1. Field of the Invention
The present invention relates to a process for preparing film of rigid rod heterocyclic liquid crystalline polymers. Such polymers include, for example, poly{[benzo(1,2- d:4,5-d')bisthiazole-2,6-diyl]1,4-phenylene}, its cis isomer or mixtures thereof (all hereinafter referred to as "PBZT"), and poly{[benzo(1,2-d:4,5-d')bisoxazole-2,6-diyl]1,4-phenylene}, its cis isomer or mixtures thereof (all hereinafter referred to as "PBO").
2. Description of the Prior Art
In the past, PBZT dope has been extruded and stretched in the uniaxial direction to produce strong, stiff, highly oriented ribbons upon coagulation. Unfortunately, these ribbons have been found to lack transverse strength due to the ordering of the structure which results in splitting upon handling. This would not present a substantial problem if the ribbons were merely used as filament wound structures. However, under certain circumstances it would be desirable to sacrifice some machine direction properties to achieve improved cross-machine strength and thus handleability.
One process for preparing films of PBZT having uniaxial orientation is described in U.S. Pat. No. 4,487,735. In that process, a solution of PBZT is formed and a film of the solution is extruded on a casting roll which rotates at a surface speed which is greater than the extrusion speed to yield a certain stretch. The film is then washed and heat treated.
There is also a method known in the art for preparing shaped articles of rigid rod heterocyclic liquid crystalline polymers having uniaxial orientation which is disclosed in U.S. Pat. No. 4,606,875. In that method, a reaction mixture containing e.g., PBZT or PBO is formed and the mixture polymerized to form a polymer solution from which shaped articles are directly formed.
Films have been blown from thermoplastic melts to generate biaxial properties. Generally, a melt of thermoplastic polymeric material is extruded upward through a tubular die pressurized with air thus expanding the molten tube of material. A blast of air is then used to solidify the melt. The resulting tube is collapsed with the aid of guide bars by a set of nip rolls which flatten the tube and confine the bubble between the nip rolls and the de face. The edges are then slit from the tube and the resulting two films are wound. These methods, however, have been considered useful only with thermoplastic polymer melts, due to their high viscosities and solids content. Unlike thermoplastic melts, blow films from polymer solutions have not been considered to be practical. That is, solvent evaporation during blowing of a solution would be expected to harden the surface thus inhibiting expansion and causing cracking. Also, normal solutions were not thought to be suitable for the stress necessary to develop orientation, whether uniaxial or biaxial.